I am the exact opposite, and tell them to make it perfect every annual. If a part is slightly suspect, change it.
Time for a new engine, cool, but don't just do the engine, make sure everything attached is perfect as well. Good time for the prop to be overhauled, how is the alternator, starter, exhaust, hoses, wiring, cables, and I don't care if the battery seems to be fine still, I replace it every 3 years. Maintenance is a lot cheaper than a failure!
Yup. I'm always amazed at the number of times I've read of alternator failures (and the occasional starter) on this site. Alternator brushes are supposed to be checked periodically, but it means removing the alternator and opening it. In the flight school most alternators got checked every 500 hours, and the brushes were shot at 1000-1200, so they got replaced at the second 500-hour check. In the G1000 172 I found them 3/4 gone at the first 500, a reflection of the extra load placed on it to drive all the avionics and stuff. I started checking them at 300 hours.
So, some guys will say, why should an airplane's alternator wear out its brushes so fast when the alternator in my car lasts for many years and miles? Well, an alternator is designed to reach its redline at the same time the engine reaches its redline, and redline represents max production capacity for the alternator. The car's engine redlines at 6000 or whatever, and it almost never gets there and when it does it isn't there very long. The airplane's alternator has a much larger pulley driving it, or it has a gearset inside the engine to drive it, and the engine can be at or near redline for very long periods. A 2700 RPM engine is often cruised at 2500, or 93% of its designed max, and so is the alternator at that high percentage, while the car's alternator might be running at 35% or so. Those little brushes are covering a lot more linear feet of slip ring movement in an airplane's 500 hours than it would in the car, and so they wear out far faster.
If the brushes get short enough they pop out of their holder and the little spring behind them runs on the slip ring, arcing madly and burning the slip ring out in very short order. Now you're replacing the entire alternator.
The older starters (like the Prestolites) have a removable brush inspection cover on them. The newer permanent-magnet starters don't, and they're designed to last to TBO. Even in the flight school, with its frequent starts, the PM starters lasted well.
And exhaust! In Canada we have an AD that applies to any airplane that get cabin heat off the exhaust system, and we are supposed to take the heat shroud off the pipe or muffler and closely check the exhaust system for cracks that will let monoxide into the cabin. Any questionable areas are to be pressure checked, and we used to do the pressure check anyway. Low pressure and soapy water. Didn't take long and was much more reliable than just eyeballing the thing. Still, some airplanes don't get looked at too well, and horror stories occasionally arise.
Some exhaust systems are prone to cracking. The 172's lycomings like to crack their exhaust stacks just below the exhaust port on the cylinder, and the muffler is prone to cracking around the stack connections. The turbo 210s have their own AD against some models for 50-hour checks on the exhaust. Turbos are hard on exhaust systems. Looking at the exhaust was one of the first things I did after getting the oil draining and so on. Cracked parts meant getting stuff ordered right away.
Looking closely at stuff during annuals is never fun. It's boring. But it's what a mechanic is paid to do.
